1. Field of the Invention
The present invention relates to a developer for use with a lithographic printing plate, and more particularly to a developer for a lithographic printing plate having a colored coating film of a negative photosensitive polymer.
2. Description of the Prior Art
Heretofore, various types of composites or mixtures have been employed as photosensitive media for negative lithographic printing, such as for example a composite prepared by mixing ammonium bichromate with albumen, gum arabic, polyvinyl alcohol or the like, a composite consisting of a diazonium salt and formaldehyde or a mixture of a diazo compound and a resin (such as novolak resin) soluble in an aqueous alkali, solution. These conventional types of photosensitive media, however, are usually not very hydrophobic and hence the affinity between the media and printing ink is is poor. Consequently, it is necessary to supply extra ink to improve fat sensitivity during manufacture of the plates. Also, since these types of sensitive materials generally possess low molecular weight, they often prove to be unable to provide sufficient image strength on a plate after exposure and development. In other words, their so-called printing resistance is poor.
An increasingly popular solution to these problems has been the, use of so-called photosensitive polymers as photosensitive materials for lithographic printing. Generally, these photosensitive polymers have better in fat sensitivity than the conventional aforementioned photosensitive materials. The adhesivity of the photosensitive materials to substrates is also good, and they possess molecular weights. Because of these characteristics, the resin which becomes the printing image after exposure and development, possesses great mechanical strength and therefore increased "printing resistance". Typical example of such photosensitive polymers is a polyether resin which has groups within the polymer of the formula (I): ##STR1## (where X and Y each represents a hydrogen atom, a halogen atom, a cyano group or a nitro group, Ar represents an aryl group which may be substituted, and n represents an integer of 1 to 2.
When this polymer is exposed to the activating light, the carbon-carbon double bond present in the side chain of the polymer molecule undergoes a photo-crosslinking reaction with the carbon-carbon double bond in the side chain of an adjoining polymer molecule to form a four-membered ring, so that the exposed area of the polymer constitutes a cross-linked network of high molecular weight structure. The exposed area of the polymer is therefore less soluble in the developing solvent than the non-exposed polymer area, because the cross-linked area of the exposed polymer is less soluble in the developing solvent that the non-exposed areas. The difference of solubility properties of unexposed polymer versus exposed polymer provides the means of forming the printing plate. In order to prepare a lithographic printing plate with such a photosensitive polymer, the following method is generally employed. First, a solution of the photosensitive polymer in a suitable solvent and additives such as a sensitizer, stabilizer, pigment, or the like is spread over a substrate such as paper, film, cloth, an aluminum plate, a zinc plate or the like which has undergone a suitable surface treatment, to form a uniform coating of the photosensitive polymer on the surface of said substrate thereby forming a printing plate. Then, after drying, a negative is tightly attached to the plate and activating light such as ultraviolet rays is irradiated over the plate, whereby the photosensitive group in the photosensitive polymer of the exposed area of the polymer undergoes a photo-crosslinking reaction to form a network of high molecular weight structure. The plate is then treated with a suitable solvent so that the polymer in the non-exposed area is swollen and dissolved in the solvent and removed from the substrate surface, thereby forming a negative-positive reversed image on the plate. The factor which is most predominant in effecting the printing plate performance is the developing characteristic of the polymeric material. During development, the non-exposed area of the photosensitive polymer is swollen and dissolved by the developing solution and thereby removed. The, the properties of the developing solution are a very important factor in the development of the exposed polymer. Generally, if the composition of the developer is of the type which promotes swelling of the resin to a very high extent, the resin in the exposed area is also affected by the solvent and is slightly swelled during development and therefore becomes slightly tacky. This partial swelling of the exposed polymer makes it very difficult to vividly reproduce the high density portion in the picture, that is, the portion with high dot area ratio, or the minute spaces between complicated patterns. The phenomenon which takes place under these conditions is generally called the "collapse" of the shadow portions of the picture. On the other hand, if the resin swelling and dissolving capacity of the developing solution is too weak, it is impossible to perfectly remove the resin in the non-exposed area from the plate surface, with the result that a residual film remains on the plate surface and hence when printing is conducted later, ink adheres to this area causing "scumming". These facts dictate that it is essential to perfectly remove the resin from the non-exposed area of the film with a developing solution of a composition which has sufficient resin dissolving power and is also capable of confining to a minimum swelling of the resin to minimize the influence of the developing solution on the exposed resin.